Polyethylene compositions for rotational molding

ABSTRACT

Polyethylene blend compositions suitable for rotomolding, rotomolded articles, and processes for rotomolding articles are provided. The polyethylene compositions include a first polyethylene having a melt index of 0.4 to 3.0 g/10 min and a density of from 0.910 to 0.930 g/cm 3 ; and a second polyethylene having a melt index of 10 to 30 g/10 min and a density of 0.945 to 0.975 g/cm 3 . The composition has a density of from 0.930 to 0.955 g/cm 3  and a melt index of 1.5 to 12 g/10 min, and the first and second polyethylenes differ in density by from 0.030 to 0.048 g/cm 3 . These compositions exhibit improved physical properties, such as Environmental Stress Crack Resistance and Izod Impact Strength.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. ProvisionalApplication No. 60/414,952, filed Oct. 1, 2002 and Ser. No. 60/424,535,filed Nov. 7, 2002, said applications incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to thermoplastic compositions of polyethylenepolymers suitable for fabrication into useful products by rotationalmolding.

BACKGROUND

Rotational molding or rotational casting, more commonly known asrotomolding, is widely used for molding hollow articles, and can be usedto mold both small and large containers, such as tanks of typically 19 Lto 57,000 L. Such rotomolded tanks are utilized in agricultural,chemical and recreational vehicle industries. Rotomolded containers areused for packaging and material handling, particularly as containerarticles for fluids or solids. Rotational molding is also used forportable toilets, instrument and battery cases, light globes, vacuumcleaner and scrubber housings, toys and garbage containers. The processis relatively less expensive and easy to use for polymer processing thanother known means and has been increasing in use.

To rotomold a part, a powdered, polymeric resin is charged inside a moldshell, which is then, typically, rotated on two axes and heated to causethe melting resin to adhere to the inside of the mold. After sufficientheating time, the mold is moved to a cooling chamber, and after cooling,the molded part is removed to begin another molding cycle. More detaileddiscussion of rotomolding may be found in Modem Plastics Encyclopedia1990, pages 317-318, and in Encyclopedia of Polymer Science andEngineering, pages 659-670 (J. Wiley & Sons, 1990).

Rotational molding primarily uses polyolefin resins, with thermoplasticpolymers of ethylene being principally used. Key properties forrotationally molded parts include appearance, and especially in the caseof containers, resistance to puncture or rupture, chemical resistanceand for extended periods of usefulness, resistance to environmentalstress cracking. Low density polyethylene (LDPE) with a density of about0.900 to about 0.925 g/cm³, linear low density polyethylene (LLDPE) witha density of about 0.926 to about 0.940 g/cm³, and high densitypolyethylene (HDPE) with a density of about 0.940 to about 0.960 g/cm³are used in rotomolding applications. LLDPE is said to be preferred forits excellent low temperature impact strength and good environmentalstress crack resistance (“ESCR”).

Compositions of polyethylene resins have been proposed to improvephysical properties, including impact strength, environmental stresscrack resistance, and chemical resistance. U.S. Pat. No. 4,438,238describes blends for extrusion processing, injection molding and filmswhere a combination of two ethylene-α-olefin copolymers with differentdensities, intrinsic viscosities and number of short chain branching per1000 carbon atoms is attributed with such physical properties. U.S. Pat.No. 4,461,873 describes ethylene polymer blends of a high molecularweight ethylene polymer, preferably a copolymer, and a low molecularweight ethylene polymer, preferably an ethylene homopolymer, forimproved film properties and environmental stress crack resistanceuseful in the manufacture of film or in blow molding techniques, theproduction of pipes and wire coating. EP 0 423 962 describes ethylenepolymer compositions particularly suitable for gas pipes said to haveimproved environmental stress cracking resistance comprising two or morekinds of ethylene polymers different in average molecular weight, atleast one of which is a high molecular weight ethylene polymer having anintrinsic viscosity of 4.5 to 10.0 dl/g in decalin at 135° C. and adensity of 0.910 to 0.930 g/cm³ and another of which is a low molecularweight ethylene polymer having an intrinsic viscosity of 0.5 to 2.0dl/g, as determined for the first polymer, and a density of 0.938 to0.970 g/cm³.

U.S. Pat. No. 5,082,902 describes blends of linear polyethylenes forinjection and rotational molding said to have reduced crystallizationtimes with improved impact strength and ESCR. The blends comprise (a) afirst polymer having a density of from 0.85 to 0.95 g/cm³ and an MI of 1to 200 g/10 min, and (b) a second polymer having a density of 0.015 to0.15 g/cm³ greater than the density of the first polymer and an MIdiffering by no more that 50% from the MI of the first polymer. U.S.Pat. No. 5,306,775 describes polyethylene blends said to have a balanceof properties for processing by any of the known thermoplasticprocesses, specifically including improved environmental stress crackresistance. These compositions have (a) low molecular weight ethyleneresins made using a chromium oxide based catalyst and having a densityat least 0.955 g/cm³ and melt index (MI) between 25 and 400 g/10 min and(b) high molecular weight ethylene copolymer resins with a density nothigher than 0.955 g/cm³ and a high load melt index (HLMI) between 0.1and 50 g/10 min.

U.S. Pat. No. 5,382,631 describes linear interpolymer polyethyleneblends having narrow molecular weight distribution (M_(w)/M_(n)≦3)and/or composition distribution (CDBI) less than 50%, where the blendsare generally free of fractions having higher molecular weight and loweraverage comonomer contents than other blend components. Improvedproperties for films, fibers, coatings, and molded articles areattributed to these blends. In one example, a first component is anethylene-butene copolymer with a density of 0.9042 g/cm³, M_(w)/M_(n) of2.3, and an MI of 4.0 dg/min and a second component is an HDPE with adensity of 0.9552 g/cm³, M_(w)/M_(n) of 2.8, and an MI of 5.0 dg/min.The blend is said to have improved tear strength characteristics.

U.S. Pat. No. 6,362,270 describes thermoplastic compositions said to beespecially suited to rotomolding applications comprising (a) a majoritycomponent that may be an ethylene interpolymer having a density greaterthan 0.915 g/cm³ and preferably a melt index of from about 2 to 500 g/10min, and (b) an impact additive that may be an ethylene interpolymerhaving a density less than 0.915 g/cm³ and melt index preferably greaterthan 0.05 g/10 min and less than 100 g/10 min. Improved physicalproperties as ascribed to these compositions include improved impactstrength and good ESCR.

There is a continuing need for polyethylene-based compositions ofimproved environmental stress crack resistance and impact strength,particularly for those that are suitable for rotomolding applications.

SUMMARY OF THE INVENTION

In accordance with the present invention, polyolefin-based blendcompositions suitable for rotomolding, rotomolded articles, andprocesses for rotomolding articles are provided.

In one embodiment, the invention provides a polyethylene compositionincluding a first polyethylene having a melt index of 0.4 to 3.0 g/10min and a density of from 0.910 to 0.930 g/cm³; and a secondpolyethylene having a melt index of 10 to 30 g/10 min and a density of0.945 to 0.975 g/cm³, wherein the composition has a density of from0.930 to 0.955 g/cm³ and a melt index of 1.5 to 12 g/10 min, and whereinthe first and second polyethylenes differ in density by from 0.030 to0.048 g/cm³.

In another embodiment, the invention provides a polyethylene compositionincluding a first metallocene-catalyzed polyethylene having a melt indexof 0.4 to 3.0 g/10 min and a density of from 0.910 to 0.930 g/cm³; and asecond metallocene-catalyzed polyethylene having a melt index of 10 to30 g/10 min and a density of 0.945 to 0.975 g/cm³, wherein thecomposition has a density of from 0.930 to 0.955 g/cm³ and a melt indexof 1.5 to 12 g/10 min, and wherein the first and second polyethylenesdiffer in density by from 0.030 to 0.048 g/cm³.

In another embodiment, the invention provides a rotomolded articleformed from or including a polyethylene composition, the polyethylenecomposition including a first polyethylene having a melt index of 0.4 to3.0 g/10 min and a density of from 0.910 to 0.930 g/cm³; and a secondpolyethylene having a melt index of 10 to 30 g/10 min and a density of0.945 to 0.975 g/cm³, wherein the composition has a density of from0.930 to 0.955 g/cm³ and a melt index of 1.5 to 12 g/10 min, and whereinthe first and second polyethylenes differ in density by from 0.030 to0.048 g/cm³.

In another embodiment, the invention provides a rotomolded articleformed from or including a polyethylene composition, the polyethylenecomposition including a first metallocene-catalyzed polyethylene havinga melt index of 0.4 to 3.0 g/10 min and a density of from 0.910 to 0.930g/cm³; and a second metallocene-catalyzed polyethylene having a meltindex of 10 to 30 g/10 min and a density of 0.945 to 0.975 g/cm³,wherein the composition has a density of from 0.930 to 0.955 g/cm³ and amelt index of 1.5 to 12 g/10 min, and wherein the first and secondpolyethylenes differ in density by from 0.030 to 0.048 g/cm³.

In another embodiment, the invention provides a process for forming arotomolded article, the process carried out by: (a) providing apolyethylene composition, the polyethylene composition including a firstpolyethylene having a melt index of 0.4 to 3.0 g/10 min and a density offrom 0.910 to 0.930 g/cm³; and a second polyethylene having a melt indexof 10 to 30 g/10 min and a density of 0.945 to 0.975 g/cm³, wherein thecomposition has a density of from 0.930 to 0.955 g/cm³ and a melt indexof 1.5 to 12 g/10 min, and wherein the first and second polyethylenesdiffer in density by from 0.030 to 0.048 g/cm³; and (b) rotomolding thecomposition to form a rotomolded article.

In another embodiment, the invention provides a process for forming arotomolded article, the process carried out by: (a) providing apolyethylene composition, the polyethylene composition including a firstmetallocene-catalyzed polyethylene having a melt index of 0.4 to 3.0g/10 min and a density of from 0.910 to 0.930 g/cm³; and a secondmetallocene catalyzed polyethylene having a melt index of 10 to 30 g/10min and a density of 0.945 to 0.975 g/cm³, wherein the composition has adensity of from 0.930 to 0.955 g/cm³ and a melt index of 1.5 to 12 g/10min, and wherein the first and second polyethylenes differ in density byfrom 0.030 to 0.048 g/cm³; and (b) rotomolding the composition to form arotomolded article.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, exceptthat each of the first and second polyethylenes has an Mw/Mn ratio offrom 1.4 to 4.0.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, exceptthat each of the first and second polyethylenes has an Mw/Mn ratio offrom 1.8 to 3.5.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, exceptthat the first polyethylene has a density of from 0.911 to 0.926 g/cm³.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, exceptthat the second polyethylene has a density of from 0.950 to 0.970 g/cm³.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, exceptthat the second polyethylene has a density of from 0.955 to 0.965 g/cm³.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, exceptthat the composition has a density of from 0.932 to 0.950 g/cm³.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, exceptthat the composition has a density of from 0.935 to 0.945 g/cm³.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, exceptthat the first and second polyethylenes differ in density by from 0.032to 0.045 g/cm³.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, exceptthat the first and second polyethylenes differ in density by from 0.035to 0.042 g/cm³.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, exceptthat the composition has a melt index I_(.2.16) of from 2 to 10 g/10min.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, whereinthe blend includes 80% to 20% by weight of the first polyethylene and20% to 80% by weight of the second polyethylene, based on the totalweight of the first and second polyethylenes.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, whereinthe blend includes 65% to 35% by weight of the first polyethylene and35% to 65% by weight of the second polyethylene, based on the totalweight of the first and second polyethylenes.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, whereinthe blend includes 55% to 45% by weight of the first polyethylene and45% to 55% by weight of the second polyethylene, based on the totalweight of the first and second polyethylenes.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, whereinthe composition has an ESCR value of at least 250 hr.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, whereinthe composition has an ESCR value of at least 500 hr.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, whereinthe composition has an ESCR value of at least 750 hr.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, whereinthe composition has an ESCR value of at least 1000 hr.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, whereinthe composition has an Izod impact strength of at least 120 kJ/m, for a3.17 mm sample at −40° C.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments, wherein atleast one of the first and second polyethylenes is a blend of two ormore polyethylene resins.

In another embodiment, the invention provides a polyethylenecomposition, a rotomolded article, or a process of forming a rotomoldedarticle, in accordance with any of the preceding embodiments except theimmediately preceding embodiment, wherein the composition includes onlythe first and second polyethylenes, except that minor amounts ofconventional additives can also be present.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a Differential Scanning Calorimetry (DSC) display of thepolymer melting temperature of two blend compositions. The solid linerepresents a blend according to the invention (3a-b in Table 1) and thebroken line represents a Comparative blend (5a-b in Table 1).

DETAILED DESCRIPTION

As indicated above, previous work has often centered on film andblowmolding applications. Thus, prior studies were often directed tofilm clarity, puncture resistance and processing characteristics forfilm processing, such extrusion and blown film processes. ESCRimprovement was also often addressed for the use of blendedpolyethylenes for such applications, especially in blowmoldingapplications. However, the prior art fails to provide polyethylene blendcompositions for the specialized technology and particular productrequirements of rotomolding. The inventive compositions surprisingly andadvantageously provide improved ESCR and significantly improved IZODimpact properties, enhancing the overall value of the inventioncompositions.

By preparing several samples of proposed blend polyethylene componentsand then subjecting blends prepared from them to analytical testing, itwas determined that peak values of ESCR are obtained when the differencein density and melt index (I_(2.16)) of the blend components were withinspecific ranges, as described herein. At smaller density differences forthe two components, ESCR was improved over single componentcompositions, but was significantly deficient to those within the rangefor the inventive compositions. Increasing the width of the densityrange between the components within the invention range increased theESCR improvement until a peak was reached in which ESCR no longerimproved and began to diminish. Examining the melting peaks of thesample blends with a differential scanning calorimeter (DSC) helpsillustrate the region in which ESCR improvements are no longer realizedby increasing the difference in densities between the two components.This is shown by the point where by further increasing the width of thedensity range, the two components no longer completely cocrystallize, asevidenced by the presence of a secondary lower melting peak in the DSCscan. When the density range was wider than that described above,evidence of loss of cocrystallizability became apparent as a secondmelting peak or shoulder began to appear in the scans. The blendsexhibiting even minimal incidence of a second shoulder had diminishedESCR improvements. See FIG. 1, and TABLE 1.

The first polyethylene of the polymer blends of the invention is alinear low density polyethylene copolymer derived from the coordinationpolymerization of principally ethylene with a minor amount of one ormore copolymerizable monomers. Particularly improved end-productproperties are obtained using such copolymers having a narrow molecularweight distribution (Mw/Mn, or “MWD”), e.g., Mw/Mn of from a lower limitof 1.4 or 1.8 or 2.0 to an upper limit of 4.0 or 3.5 or 3.0, with rangesfrom any lower limit to any upper limit being contemplated. Suitablecomonomers include C₃-C₂₀ alpha-olefins, preferably C₃-C₈, C₅-C₂₀ cyclicolefins, preferably C₇-C₁₂ cyclic olefins, C₇-C₂₀ vinyl aromaticmonomers, preferably styrene, and C₄-C₂₀ geminally disubstitutedolefins, preferably isobutylene. The most preferred comonomers includepropylene, 1-butene, 1-hexene, 4-methyl-1-pentene and 1-octene. Thedensity of the copolymer is determined largely by comonomer content andtypically ranges from 0.910 or 0.911 g/cm³ to 0.930 or 0.926 g/cm³, withranges from any lower limit to any upper limit being contemplated. Someamount of long-chain branching may be present, but the densitylimitations are largely due to the presence of comonomer. These ethylenecopolymers are of higher molecular weight than the second polyethyleneof the blends, as shown by a melt index I_(2.16) as measured accordingto ASTM D1238, condition 190° C. 2.16 kg (formerly condition “E”), offrom about 0.4 to about 3.0 g/10 min. This molecular weight range isapproximately equivalent to an intrinsic viscosity (in decalin at 135°C.) of from about 1.2 to about 1.7 dl/g.

The second polyethylene of the polymer blends of the invention is a highdensity polyethylene of similar Mw/Mn to the first polyethylene, i.e.,Mw/Mn of from a lower limit of 1.4 or 1.8 or 2.0 to an upper limit of4.0 or 3.5 or 3.0, with ranges from any lower limit to any upper limitbeing contemplated, but with lower molecular weight. It will be derivedfrom ethylene and, optionally, minor amounts of any of the comonomerslisted above for the first polyethylene. The density can be from a lowerlimit of 0.945 or 0.950 or 0.955 g/cm³ to an upper limit of 0.975 or0.970 or 0.965 g/cm³, with ranges from any lower limit to any upperlimit being contemplated. The lower molecular weight is shown by a meltindex I_(2.16) as measured according to ASTM D1238, condition 190° C.,2.16 kg, of from 10 to 150 g/10 min. This molecular weight range isapproximately equivalent to an intrinsic viscosity (in decalin at 135°C.) of from about 0.9 to about 1.2 dl/g. The melt index I_(2.16) of thesecond polyethylene can range from a lower limit of 10 or 12 or 14 g/10min to an upper limit of 150 or 100 or 50 or 30 g/10 min, with rangesfrom any lower limit to any upper limit being contemplated.

Industrial methods of producing the polyethylene components of theinvention are well known in the art as is exemplified in the referencescited above. Any such method capable of producing polyethylene polymercomponents according to the invention will be suitable. Such methodsinclude gas phase, liquid phase (or solution), and slurry phasepolymerization processes, either alone or in combination. By alone,reference is made to series or serial production in a single reactor orin more than one reactor. Reactor blends will also be suitable, such asby the use of mixed catalysts or mixed polymerization conditions in asingle reactor. Gas phase processes are particularly suited in view ofeconomic advantages. Such processes use supported catalysts and areconducted in polymerization reactors under gas phase conditions suitablefor linear low density ethylene copolymers prepared by coordinationpolymerization. Illustrative examples may be found in U.S. Pat. Nos.4,543,399, 4,588,790, 5,028,670, 5,352,749, 5,382,638, 5,405,922,5,422,999, 5,436,304, 5,453,471, 5,462,999 and 5,463,999, andInternational applications WO 94/28032, WO 95/07942 and WO 96/00245.These processes use either traditional Ziegler-Natta catalysts or laterorganometallic catalysts characterized as having essentially singlepolymerization sites due to the arrangement of ancillary ligands on orabout the metal center. Metallocene catalysts are representative “singlesite catalysts” and are preferred in this invention for their ability toproduce narrow molecular weight distribution polyolefins. Typically, theprocesses are conducted at temperatures of from about −100° C., to 150°C., more typically from about 40° C to 120° C., at pressures up to about7000 kPa, typically from about 690 kPa to 2415 kPa. Continuous processesusing fluidized beds and recycle streams as the fluidizing medium arepreferred.

Slurry polymerization processes are suitable for both components andparticularly suited for the high density components of the invention.These processes are typically described as those in which thepolymerization medium can be either a liquid monomer, like propylene, ora hydrocarbon solvent or diluent, advantageously aliphatic paraffin suchas propane, isobutane, hexane, heptane, cyclohexane, etc. or an aromaticone such as toluene. Slurry solids typically include the forming polymerand inert carrier-supported catalysts. Catalysts are typicallyZiegler-Natta, and/or one or more single site catalysts, such asmetallocenes. The polymerization temperatures may be those consideredlow, e.g., less than 50° C., typically 0° C.-30° C., or may be in ahigher range, such as up to about 150° C., typically from 50° C. up toabout 80° C., or at any ranges between the end points indicated.Pressures can vary from about 100 to about 700 psia (0.76-4.8 MPa).Additional description is given in U.S. Pat. Nos. 4,182,810, 5,274,056,6,319,997, 6,380,325, 6,420,497, WO 94/21962 and WO 99/32531.

The polyethylene blend compositions in accordance with the presentinvention can include the first polyethylene in an amount of from alower limit of 20 or 35 or 45 wt % to an upper limit of 80 or 65 or 55wt %, based on the total weight of the first and second polyethylenes,with ranges from any lower limit to any upper limit being contemplated.Similarly, the polyethylene blend compositions in accordance with thepresent invention can include the second polyethylene in an amount offrom a lower limit of 20 or 35 or 45 wt % to an upper limit of 80 or 65or 55 wt %, based on the total weight of the first and secondpolyethylenes, with ranges from any lower limit to any upper limit beingcontemplated.

Additionally, either or both of the first polyethylene and the secondpolyethylene can be a sub-blend of two or more polyethylenes so long asthe sub-blend has the properties described herein.

Although the description herein focuses on first and secondpolyethylenes, in some embodiments, the polyethylene blend compositioncan further include additional polymeric components, includingadditional polyethylenes, provided that the overall blend compositionhas the recited properties.

The weight percentages recited herein for the first and secondpolyethylene components are based on the total weight (100%) of thefirst and second polyethylene components.

The blend can have a density of from a lower limit of 0.930 or 0.932 or0.935 g/cm³ to an upper limit of 0.955 or 0.950 or 0.945, with rangesfrom any lower limit to any upper limit being contemplated.

The blend can have a difference in the density of the first and secondpolyethylenes of from a lower limit of 0.030 or 0.032 or 0.035 g/cm³ toan upper limit of 0.048 or 0.045 or 0.042 g/cm³, with ranges from anylower limit to any upper limit being contemplated.

The melt index of the blend can be from a lower limit of 1.5 or 2.0 g/10min to an upper limit of 12 or 10 or 8 g/10 min.

Polyethylene blend compositions of the invention show ESCR values ofgreater than 250 or greater than 500 or greater than 750 or greater than1000 hr.

Polyethylene blend compositions of the invention show Notched IzodImpact values (−40° C., 3.17 mm thick sample) of greater than 120 kJ/m.

Additives may be used as needed. Typical additives include one or moreof antioxidants, anti-static agents, UV stabilizers, foaming agents,processing aids, nucleating agents, nanocomposites, fiber reinforcementsand pigments. Illustrative pigments or colorants include titaniumdioxide, carbon black, cobalt aluminum oxides such as cobalt blue, andchromium oxides such as chromium oxide green. Pigments such asultramarine blue, which is a silicate. Phthalocyanine blue and ironoxide red will also be suitable. Such are typically used an amounts from0 wt % to not more than about 15 wt %, based on the total weight of thefirst and second polyethylene components.

In accordance with the invention, a polyolefin-based, resin blend aspreviously described, is rotomolded. To this end, the resins, with orwithout additives, may be extrusion blended, pelletized and ground to apowder, typically of 35 U.S. mesh (500 μm) which means that the averageparticle size is typically 60 U.S. mesh (250 μm). A suitable extrusionblending temperature is typically about 190 to 210° C. Thereafter, thepowder is placed inside a hollow mold, which is typically rotated on twoaxes and heated inside an oven. The powder is heated for a sufficienttime and at a temperature adequate to melt the thermoplasticconstituents of the powder blend, during the rotomolding. The time andtemperature used depend upon factors including the thickness of the partbeing rotomolded and thermal sensitivity of the constituents, and oneskilled in the art can readily determine suitable processing conditions.As applied to the polyethylene resin blends of the invention, a partthickness of about ⅛″ (0.3175 cm), an oven temperature setting rangingfrom about 550° F. to 650° F. (287.8 to 343.3° C.) for about 10 to 20minutes will typically provide sufficient melting conditions.

EXAMPLES

Notched IZOD tests were conducted in accordance with ASTM D-256, MethodA.

Mz, Mw and Mn can be measured using gel permeation chromatography (GPC),also known as size exclusion chromatography (SEC). This techniqueutilizes an instrument containing columns packed with porous beads, anelution solvent, and detector in order to separate polymer molecules ofdifferent sizes. In a typical measurement, the GPC instrument used is aWaters chromatograph equipped with ultrastyro gel columns operated at145° C. The elution solvent used is trichlorobenzene. The columns arecalibrated using sixteen polystyrene standards of precisely knownmolecular weights. A correlation of polystyrene retention volumeobtained from the standards, to the retention volume of the polymertested yields the polymer molecular weight.

Average molecular weights M can be computed from the expression:$M = \frac{\sum\limits_{i}{N_{i}M_{i}^{n + 1}}}{\sum\limits_{i}{N_{i}M_{i}^{n}}}$where N_(i) is the number of molecules having a molecular weight M_(i).When n=0, M is the number average molecular weight Mn. When n=1, M isthe weight average molecular weight Mw. When n=2, M is the Z-averagemolecular weight Mz. The desired MWD function (e.g., Mw/Mn or Mz/Mw) isthe ratio of the corresponding M values. Measurement of M and MWD iswell known in the art and is discussed in more detail in, for example,Slade, P. E. Ed., Polymer Molecular Weights Part II, Marcel Dekker,Inc., NY, (1975) 287-368; Rodriguez, F., Principles of Polymer Systems3rd ed., Hemisphere Pub. Corp., NY, (1989) 155-160; U.S. Pat. No.4,540,753; Verstrate et al., Macromolecules, vol. 21, (1988) 3360; andreferences cited therein.

Environmental Stress Crack Resistance (ESCR) (bent strip) is determinedin accordance with ASTM D 1693, condition B, 10% IGEPAL™. IGEPAL™ is anonylphenoxy poly(ethylenoxy)ethanol surfactant available from RhonePolenc, Cranbury, N.J. All ESCR values cited herein are ASTM D 1693condition B, 10% IGEPAL™ F50 values, and are given in units of hours.

Polymer density (g/cm³) is determined using a compression molded sample,cooled at 15° C. per hour and conditioned for 40 hours at roomtemperature according to ASTM D1505-68 and ASTM D1928, procedure C.

Polymer melt flow rates can be determined at 190° C. according to ASTMD-1238. I_(21.6) is the “flow index” or melt flow rate of the polymermeasured according to ASTM D-1238, condition 190° C., 21.6 kg, andI_(2.16) is the “melt index” or melt flow rate of the polymer measuredaccording to ASTM D-1238, condition 190° C., 2.16 kg. The ratio ofI_(21.6) to I_(21.6) is the “melt flow ratio” or “MFR”. The melt flowrate I_(21.6) is also sometimes termed the “high load melt index” orHLMI. Melt flow rates are reported in units of grams per 10 minutes(g/10 min) or equivalently decigrams per minute (dg/min).

Examples 1-3, Comparative Examples 1-5

The examples shown in Table 1 were prepared generally in accordance withthe examples in U.S. Pat. No. 5,382,631, except where noted. Azirconocene activated with alumoxane on a silica support, 12 wt %methylalumoxane and 3.5 wt % zirconium, was used as polymerizationcatalyst in a gas phase reactor operated at about 185° F. (85° C.), witha gas phase consisting of 70 vol % ethylene, 0.5-2.0 vol % hexene,200-800 parts per million hydrogen, with remainder being nitrogen. Fromabout 50 to 75 pounds (22.6 to 33.9 kg) per hour were produced in eachpolymerization run.

Table 1 illustrates the invention in examples 1a-b through 3a-3b, withcomparative examples Comp 1 through Comp 5a-b. Each “a” row illustratesa first polyethylene component and each “b” row illustrates a secondpolyethylene component. The column “Δdensity” provides the difference indensity of the two components for each illustrated blend. Comp 1illustrates a comparative single polyethylene component within thedensity and melt index range typical for rotomolding compositions. Comp2 illustrates a comparative blend where the melt indexes of the twocomponents are approximately equal and the densities are such that thedifference is less than 0.030 g/cm³ but the average is the same as thatof Comp 1. Comp 3 illustrates a comparative blend where the highmolecular weight first and low molecular weight second polyethyleneseach have the same density as Comp 1. Comp 4 illustrates a comparativeblend where densities are the same but the molecular weight of the highmolecular weight fraction and the blend composition have been increased.Comp 5 illustrates a comparative blend where the high molecular weightfirst polyethylene component has a density below 0.910 g/cm³. As isreadily apparent, the invention examples each have excellent ESCR andthe comparative examples each do not.

TABLE 1 Melt Index I_(2.16) Δ (g/ Density density Mw/ ESCR, Example Wt %10 min) (g/cm³) (g/cm³) Mn F₅₀ (hr) 1a 48.4 0.86 0.919 2.43 1b 51.6 14.00.950 3.34 1a/1b Blend 100 2.7 0.935 0.031 >1000 2a 45 0.86 0.919 2.432b 55 14.0 0.950 3.34 2a/2b Blend 100 3.1 0.936 0.031 >1000 3a 38.5 0.460.911 2.50 3b 61.5 14.0 0.950 3.34 3a/3b Blend 100 2.9 0.935 0.039 >1000Comp 1 100 3.05 0.935 2.82 <220 Comp 2a 55.5 3.0 0.947 2.87 Comp 2b 45.52.88 0.920 2.43 Comp 2a/2b Blend 100 3.0 0.935 0.027 <180 Comp 3a 580.97 0.934 2.93 Comp 3b 42 14 0.934 2.58 Comp 3a/3b Blend 100 3.0 0.9340 <250 Comp 4a 48 3.0 0.935 2.82 Comp 4b 52 14 0.934 2.58 Comp 4a/4bBlend 100 7.6 0.935 0.01 <100 Comp 5a* 30 1.2 0.900 2.0 Comp 5b 70 140.950 3.34 Comp 5a/5b Blend 100 7.6 0.935 0.050 <100 *Commercialethylene-based hexene plastomer (Exact ™ 3132, ExxonMobil Chemical)

As further illustrated in FIG. 1, Comparative example 5a-b (dotted line)exhibits dual melting temperatures by DSC, at 95.9° C. and 127.7° C.Invention example 3a-b (solid line) exhibits a single meltingtemperature at 128.4° C. Both of these compared examples have the samedensity, indicative of essentially equivalent average comonomer content,yet cocrystallization is effectively achieved only with the inventionblend.

Examples 4-5, Comparative Example 6

The examples 4 and 5 of Table 2 were prepared by melt blending twoselected components in accordance with the invention. The high molecularweight first polyethylene used was a commercial film gradeethylene-based hexene copolymer (Exceed™ 1023CA, ExxonMobil ChemicalCompany) and the low molecular weight second polyethylene was producedin an ExxonMobil Chemical commercial slurry loop reactor utilizing asilica supported zirconocene activated with methylalumoxanepolymerization catalyst under conditions used to produce high densitypolyethylene. The blend and comparative examples of Table 2 additionallyincluded approximately equivalent amounts of additives: Irganox™ 3114primary antioxidant (CIBA); Irgafos™ 168 secondary antioxidant (CIBA);and acid neutralizer (zinc stearate, or equivalent). The density, meltindex I_(2.16) and Mw/Mn of both are shown below. Comp 6 is acomparative example illustrating the properties of a commercial singlecomponent LLDPE resin sold for rotational molding use and described ashaving excellent ESCR and toughness. Examples 4 and 5 additionallyexhibited low High Load Melt Indexes, I_(21.6), ASTM-D 1238 (190° C.,21.6 kg) of 17.9 each as compared to the Comp 6 sample value of 82.5.

As can be seen in Table 2, both the ESCR and Notched Izod propertiesshow significant improvements over the comparison composition. The ESCRvalues for invention examples 4 and 5 are reported as >288 hr in thatthe testing was stopped at 288 hours. It can be reasonably anticipatedthat the total performance would have matched that of the Table 1invention samples if the testing were run for as long a period of time.Additional data obtained indicated that the invention blends of Table 2exhibited surprisingly improved ARM (drop weight) impact strength,flexural modulus and tensile break stress as compared to the comparisoncomposition.

TABLE 2 Notched Izod, −40° C. Melt Index I_(2.16) Density Δ densityESCR, F₅₀ (ft-lb/in) (kJ/m) Example Wt % (g/10 min) (g/cm³) (g/cm³)Mw/Mn (hr) 3.17 mm** 6.35 mm** 4a 55 1.0 0.923 2.9 4b 45 21.2 0.958 2.74a/4b 100 3.0 0.939 0.035 3.2 >288 2.58 1.62 Blend 137.8 86.5 5a 45 1.00.923 3.1 5b 55 21.2 0.958 5a/5b 100 3.7 0.942 0.035 3.1 >288 1.97 1.5Blend 105.2 80.1 Comp 6* 100 3.3 0.939 3.6 52 1.94 1.20 103.6 64.1*Commercial LLDPE of ethylene and hexene (LL 8460, ExxonMobil Chemical)**sample thickness

Various tradenames used herein are indicated by a ™ symbol, indicatingthat the names may be protected by certain trademark rights. Some suchnames may also be registered trademarks in various jurisdictions.

All patents, test procedures, and other documents cited herein,including priority documents, are fully incorporated by reference to theextent such disclosure is not inconsistent with this invention and forall jurisdictions in which such incorporation is permitted.

1. A polyethylene composition comprising: (a) a first polyethylenehaving a melt index of 0.4 to 3.0 g/10 min and a density of from 0.910to 0.930 g/m³; and (b) a second polyethylene having a melt index of 10to 30 g/10 min and a density of 0.945 to 0.975 g/cm³, wherein thecomposition has a density of from 0.930 to 0.955 g/cm³ and a melt indexof 1.5 to 12 g/10 min, wherein the first and second polyethylenes differin density by from 0.030 to 0.048 g/cm³, wherein at least one of thefirst and second polyethylenes comprises a blend of two or morepolyethylene resins.
 2. A rotomolded article comprising a polyethylenecomposition, the polyethylene composition comprising: (a) a firstpolyethylene having a melt index of 0.4 to 3.0 g/10 min and a density offrom 0.910 to 0.930 g/cm³; and (b) a second polyethylene having a meltindex of 10 to 30 g/10 min and a density of 0.945 to 0.975 g/cm³,wherein the composition has a density of from 0.930 to 0.955 g/cm³ and amelt index of 1.5 to 12 g/10 min, wherein the first and secondpolyethylenes differ in density by from 0.030 to 0.048 g/cm³, andwherein at least one of the first and second polyethylenes comprises ablend of two or more polyethylene resins.